The present invention relates to cross-heads for extruders or similar pressure sources of flowable polymeric materials and more particularly to improvements in such cross-heads aimed at providing a balanced flow at the die through which the material is to be forced at the cross-head output.
While in some applications it is possible to provide extruders and the like with dies which lie axially to the extruder it is often more efficient to provide the extruder with a cross-head so that extrusion through the firming die takes place at right angles to the extruder. A difficulty encountered, however, in using a cross-head is the fact that in bending the stream of material unequal pressures are introduced in the stream cross-section resulting in unequal amounts of material being forced through different portions of the die. This causes distortion of the part being shaped unless the die is designed unsymmetrically to exactly compensate for the distorted pressure pattern, a very difficult achievement.
Ideally the material leaving the output end of the cross-head and reaching the die would have uniform pressure distribution and would thereby eliminate the problem. The very viscous nature of the material normally used in extrusion processes would require a very long reach of laminar flow before pressure equilibrium could occur. While less viscous materials would allow pressure equilibrium to be achieved in a shorter distance such less viscous materials tend to sag upon leaving the die, creating yet another type of distortion.